In the cockpit of an aircraft and in a control tower of an airport, it is important to have good visibility of the knobs on a control panel. Internally illuminated panels are known to improve the visibility of the knobs, especially in a darkened environment. To further improve the visibility, knobs of translucent construction are employed with these internally illuminated panels.
A conventional knob of translucent construction has a transparent or translucent body made of plastic or acrylic, which is coated with a translucent reflecting material. Coated over the translucent reflecting material is an opaque material. An indicator line is formed on the knob by engraving through the opaque material, thereby exposing the translucent reflecting material. A contrast in colors between the opaque and reflecting materials is desirable, e.g., black for the opaque material and white for the reflecting material. When light is transmitted through the translucent body of the knob, the exposed reflecting material, which forms the pointer, is illuminated. Such a knob is simple and inexpensive to manufacture.
The internally illuminated panel typically includes a flat, light-transmitting core that is coated with an opaque layer. An annular area of the opaque layer is removed, which exposes an underlying area of the light-transmitting core. The exposed annular area is covered by the body of the knob. Light, provided by a source of illumination on the opposite side of the core, is transmitted through the exposed area and into the body of the knob, whereby the pointer on the knob is illuminated. Knobs of translucent construction and internally illuminated panels are well known to those skilled in the art. See, for example, U.S. Pat. No. 2,763,231 issued to Edwin Neugass on Sep. 18, 1956.
A problem associated with internally illuminated panels arises when light leaks from a gap located between the knob of translucent construction and the core of the panel. The leakage causes a bright halo of light around the knob, which is a source of concern and disturbance, especially to flight crews during darkened flying conditions. Not only is the halo a source of nuisance, but it also degrades the appearance and readability of cockpit instruments.
Attempts have been made to reduce such light leakage. In U.S. Pat. No. 2,699,141 issued to Edward Gaguski on Jan. 11, 1955, a bevel is cut into the underside of the core of the internally illuminated panel. The beveled surface is coated with a reflecting material. Light is reflected off the beveled surface and into the body of the knob, whereby the pointer is illuminated. In an alternate embodiment, a bevelled recess is cut into the body of the knob. The bevelled surface is coated with a light-reflective material. The knob has its lower portion inserted into an aperture in the core. Light supplied by the internally illuminated panel is reflected into the upper portion of the body to illuminate the pointer. See also U.S. Pat. No. 4,163,428 issued to Masao Ishikawa on Aug. 7, 1979, in which a source of illumination is disposed within the knob. Although the Ishikawa knob and the alternate embodiment disclosed by Gaguski reduce light leakage, they require specially constructed knobs.
Among other disadvantages associated with internally illuminated panels and its knobs, the underside of the knob must be positioned from the core so as not to rub against the opaque paint. The step of placing a small gap between the knob and the core is time consuming.
Therefore, it is an object of the present invention to reduce light leakage in an internally illuminated panel that employs conventional knobs of translucent construction.
It is another object of the present invention to facilitate the step of mounting the knob onto the internally illuminated panel.